TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 199 



limb or future left ventricle, Ven. S, is already larger than the right limb. The 

 external groove, F, which marks the boundary between the two auricles is 

 clearly shown by the section. The trabecular structure of the ventricles is well 

 developed, and affords a diagnostic path by which the ventricles, if they are cut, 

 may be easily recognized in sections. The development of the trabeculae corre- 

 sponds to the formation of blood sinusoids of the heart. The trabeculae consist 

 of young muscle cells, and each bundle of cells is closely invested by the endo- 

 thelium of the heart. The blood thus circulates freely between the trabeculae, 

 but remains, as in every blood-channel, separated by the endothelium from the 

 neighboring tissue. The tissues of the heart are thus enabled to get their nour- 

 ishment from the blood circulating through the organ. The sinusoidal type of 

 circulation which we here encounter appears in the heart of all vertebrate em- 

 bryos, and is the permanent form of circulation in the frog. In mammals, on 

 the other hand, although the sinusoidal circulation is kept throughout life and 

 the ventricles always have their trabecular structure, yet we find, in addition, the 

 development of a true capillary circulation to supplement the sinusoidal. This 

 capillary circulation is supplied by the coronary arteries, and develops compara- 

 tively late. Between the auricles and the ventricles the heart is narrow. This 

 constricted region is known as the auricular canal. A broad partition, 5. a, c, 

 divides the cavity of the auricular canal into right and left channels, forming 

 open vessels between the auricles and ventricles. From the lower edges of these 

 channls flaps of tissue project into the ventricles. The flaps are the anlages of 

 the atrio-ventricular valves. 



Sections through the Anterior Limbs to Show the Brachial Plexus. — Figure 

 121 was drawn from a single section, except that the nerves in the limbs repre- 

 sent a reconstruction from several adjacent sections. The limb-bud, A. L, pro- 

 jects freely from the side of the body, is covered by ectoderm, Ec, and filled with 

 a very dense tissue, the cells of which show no very clear histological differentia- 

 tion. The spinal cord, Sp. c, is fairly well advanced in its development at this 

 level, and shows a darker, inner layer, Epen, a middle, gray layer, cin, and an 

 outer neuroglia, Ec. gl. The cord is completely surrounded by the developing 

 pia mater, which is quite thin, but highly vascular. The ganglia are cut almost 

 symmetrically on the two sides and show their dorsal roots. The descending 

 trunk from each ganglion is joined by the ventral roots, V. R, which arise from 

 the ventral zone of the cord in several bundles which unite about the same time 

 with both one another and the dorsal root to form the main nerve-trunk, N.8, 

 which enters into the formation of the brachial plexus. Just after the junction 

 of the two roots the nerve gives off a branch which runs obliquely dorsalwards 

 into the anlage of the dorsal muscles. Muse. This branch is, of course, the dorsal 



